Solar Panel Calculator
Sizing a solar system is a three-step calculation: how much energy you use, how much sun your roof gets, and how much each panel produces. This solar panel calculator combines all three.
The 0.75 performance ratio accounts for real-world losses: inverter efficiency, temperature, dust and wiring.
Enter valid values to see the results.
Formula
Peak sun hours are the equivalent hours of full 1,000 W/m² sunlight per day: 3–4 in northern Europe, 4–5 in mid-latitudes, 5.5–7 in the southwest US and Mediterranean. One 400 W panel needs about 2 m² of roof.
How to measure
- Find your monthly kWh on your electricity bill — use a 12-month average, not the best month.
- Look up peak sun hours for your location (PVGIS for Europe, NREL maps for the US).
- Check usable roof area: south-facing (northern hemisphere), unshaded, minus skylights and chimneys.
Worked example
Practical tips
- Size for your annual use, not your best month — net metering credits seasonal differences in many regions.
- Shading on even one panel can cut a string’s output by 30%+; consider microinverters for complex roofs.
- Panel wattage keeps rising: 400–450 W is the current residential standard.
- Get three installer quotes — prices for identical systems commonly differ by 20–30%.
Waste allowance
The 0.75 performance ratio already includes system losses. Add 5–10% capacity only if you plan to add an EV or heat pump soon.
Peak sun hours by region (annual average)
| Region | Sun hours/day | kWh per kW per year |
|---|---|---|
| Northern Europe | 2.5–3.5 | 700–950 |
| Central Europe / northern US | 3.5–4.5 | 950–1,250 |
| Mediterranean / California | 4.5–5.5 | 1,250–1,500 |
| Southwest US / Australia | 5.5–7.0 | 1,500–1,900 |
Common mistakes
- 1 Sizing from one summer bill — winter use with electric heating can be double.
- 2 Ignoring shading: a chimney shadow at 4 pm costs more than the math suggests.
- 3 Using lab panel output without the 0.75 real-world factor, oversizing expectations by a third.
- 4 Forgetting future loads: an EV adds 2,000–4,000 kWh per year to the house.
Frequently asked questions
How many solar panels do I need for 1,000 kWh per month?
With 4.5 peak sun hours and 400 W panels: about 25 panels (a 10 kW system). In sunnier regions 20 panels suffice; in cloudy ones, 30+.
How much roof space do solar panels need?
About 2 m² (21 ft²) per modern 400 W panel, or roughly 18–20 m² per 4 kW of system size.
What are peak sun hours?
The number of hours per day when sunlight averages full strength (1,000 W/m²). Five peak sun hours means your panels produce their rated wattage for the equivalent of 5 full hours.
How much does a home solar system cost?
Before incentives, roughly $2.5–3.5 per watt installed in the US ($25k–35k for 10 kW), less in many other markets. Tax credits and rebates commonly cut 20–30%.
Do solar panels work in winter or cloudy weather?
Yes, at reduced output: 10–25% of rated power under heavy cloud. Annual production figures already average seasons, which is why sun-hour data matters more than weather on a given day.